Knowledge of the ambient pressure represents an important variable for operation of internal combustion engines. In accordance with a first known alternative the ambient pressure is measured directly with the aid of an ambient pressure sensor. In accordance with a further alternative, in pressure-controlled systems, i.e. a pressure sensor is located in the intake pipe of the internal combustion engine, the ambient pressure is estimated with the aid of a prediction process. For internal combustion engines with lift changeover (for example two-stage) attempts are further made, even with “unthrottled” operation of the internal combustion engine, to still maintain a specific pressure difference to the environment. This pressure difference is used for venting air from the tank or for filling the brake servo. The previous prediction methods however did not take account for example in full-load operation of the internal combustion engine of this residual pressure difference.
The ambient pressure is detected in pressure-controlled systems of the prior art in the following manner. Before the internal combustion engine is started the intake pipe pressure corresponds to the ambient pressure of the internal combustion engine. In the area close to full load the ambient pressure is estimated based on the measured intake pipe pressure with an almost open throttle flap. With a closed throttle flap in the overrun mode of the internal combustion engine a specific pressure arises in the intake pipe after a certain time. From this pressure the ambient pressure is likewise estimated, whereas this estimation however, because of manufacturing tolerances of the throttle flap, is very imprecise.